Originally Posted by dhd40

from mugaliens´ post:
All particles must obey E2 = p2c2 + m2c4

isn´t that: E²= m²v²c²+m²c⁴

Then again from mugaliens´ post:
For a photon, we declare 0 rest mass

ergo: E²=0 ???

And as I said somewhere else: what´s the meaning of "rest mass"? What´s the meaning of "relativistic mass"? There´s no rest mass, and no relativistic mass, there´s only mass, invariant, independent of the frame of reference
And yes, for a photon E=h*f
But why do we need any mass-considerations concerning photons?